The present invention relates to dielectric waveguide pairs made by plasma enhanced chemical vapour deposition (PECVD) upon planar substrates. Such deposition involves depositing and patterning material to form cores of material having a slightly greater refractive index than that of buffer layer material upon which those cores are supported and that of cladding material with which those cores are covered. Typically these waveguide pairs are designed for guiding light in the 1 to 2 .mu.m for optical communication systems.